This invention relates to the welding of protected metal parts, for example sheetmetal sections covered with a material which can volatilize at relatively low temperatures, in any case, lower than the temperature of welding or brazing the parts, for example zinc, synthetic resins, etc. . .
Generally, to avoid deteriorating or eliminating the coating from welded parts, welding has been renounced and other assembly techniques have been used, such as bolting or clipping, or welding has been performed but a later operation is performed to repair and re-cover the deteriorated surfaces.
These methods have their disadvantages. Assembly operations such as bolting and clipping are very expensive and do not provide a really leaktight joint which is required for a certain number of applications. The later operation of re-covering is an additional production operation, and if the later treatment involves dipping, as for galvanizing and certain paint coating techniques, there may be problems because of the large dimensions of the assemblies with their inherent handling difficulties.
It has already been proposed, with resistance welding pieces which are coated with protective insulating coating, to spray onto the parts to be welded, in the zone where the electrodes are working, a fluid containing a substance which conducts electricity and which can be cooled, so as to simultaneously cool and clean the welding electrodes. As fluid, cooled water is used. This method is applicable to resistance welding and may in no case be used for electric arc welding. Moreover, the use of water, simply poured into the gap between the electrode and the part, would tend to have adverse effects rather than beneficial effects since not only would it disturb the proper function of the arc, but, because of the limitation imposed by the cooling temperature, it would require large quantities of water which, when cooling the bodies of the parts, would considerably disturb the melting metal, and would even be able to inhibit its formation.